[Bug]: Can't build on macOS with GCC 15

[OleksandrKvl]

Describe the issue

Failed to build abseil via Conan or directly from git using GCC-15 installed via brew. The error is:

[build] FAILED: [code=1] absl/time/CMakeFiles/time_zone.dir/internal/cctz/src/time_zone_format.cc.o 
[build] /opt/homebrew/bin/g++-15  -I/Users/alex/Documents/src/abseil-cpp -g -arch arm64 -Wall -Wextra -Wcast-qual -Wconversion-null -Wformat-security -Wmissing-declarations -Wnon-virtual-dtor -Woverlength-strings -Wpointer-arith -Wundef -Wunused-local-typedefs -Wunused-result -Wvarargs -Wvla -Wwrite-strings -DNOMINMAX -MD -MT absl/time/CMakeFiles/time_zone.dir/internal/cctz/src/time_zone_format.cc.o -MF absl/time/CMakeFiles/time_zone.dir/internal/cctz/src/time_zone_format.cc.o.d -o absl/time/CMakeFiles/time_zone.dir/internal/cctz/src/time_zone_format.cc.o -c /Users/alex/Documents/src/abseil-cpp/absl/time/internal/cctz/src/time_zone_format.cc
[build] In file included from /opt/homebrew/Cellar/gcc/15.2.0_1/include/c++/15/ext/string_conversions.h:45,
[build]                  from /opt/homebrew/Cellar/gcc/15.2.0_1/include/c++/15/bits/basic_string.h:4444,
[build]                  from /opt/homebrew/Cellar/gcc/15.2.0_1/include/c++/15/string:56,
[build]                  from /Users/alex/Documents/src/abseil-cpp/absl/time/internal/cctz/include/cctz/time_zone.h:27,
[build]                  from /Users/alex/Documents/src/abseil-cpp/absl/time/internal/cctz/src/time_zone_format.cc:30:
[build] /opt/homebrew/Cellar/gcc/15.2.0_1/include/c++/15/cstdlib:147:11: error: 'at_quick_exit' has not been declared in '::'
[build]   147 |   using ::at_quick_exit;
[build]       |           ^~~~~~~~~~~~~
[build] /opt/homebrew/Cellar/gcc/15.2.0_1/include/c++/15/cstdlib:170:11: error: 'quick_exit' has not been declared in '::'
[build]   170 |   using ::quick_exit;
[build]       |           ^~~~~~~~~~

Steps to reproduce the problem

Build abseil via CMake.

What version of Abseil are you using?

6c1f120

What operating system and version are you using?

macOS Sequoia 15.7.4

What compiler and version are you using?

gcc-15 (Homebrew GCC 15.2.0_1) 15.2.0

What build system are you using?

CMake 4.3.0

Additional context

Google suggested that the error might be related to _XOPEN_SOURCE macro being defined. The build succeeded when I commented it out here:

abseil-cpp/absl/time/internal/cctz/src/time_zone_format.cc

Line 25 in 9cc7e8d

#define _XOPEN_SOURCE 500 // Exposes definitions for SUSv2 (UNIX 98).